1. Field of the Invention
The present invention relates to a curable and foamable polyolefinic resin composition, a cured foam of polyolefinic resin to be formed from the composition, and a method for producing the cured foam.
More precisely, the invention relates to a cured foam of polyolefinic resin which has the advantages of high heat resistance, good workability into complicated articles, good workable characteristics at high temperatures, high pressure resistance at high temperatures, and well-controllable flexibility, and which can therefore be subjected to broad secondary working to give various articles of many applications, and also relates to a curable and foamable polyolefinic resin composition which is favorably and continuously formed into the cured foam.
2. Description of the Related Art
Cured foams of resin consisting essentially of polypropylenic resin and those consisting essentially of polyethylenic resin have many applications in various fields, as having the advantages of good workability, good buffering ability, good heat insulation, and high heat resistance. For example, polypropylenic resin foams are laminated with a skin layer of, for example, polyvinyl chloride sheets, olefinic (elastomeric) resin sheets (TPO sheets) or woven or knitted fabrics, and worked in various working methods into upholstery for cars, such as door parts, instrument panels, console boxes, sheet-back garnishes, etc.
Those cured foams of polyolefinic resin are produced in a process comprising molding a resin composition that contains a curing agent and a chemical foaming agent, curing it and then foaming it. One curing method for the process comprises exposing a continuous sheet of a foamable resin composition to ionizing radiations. Low-density polyethylenic resin as prepared in a high-pressure method is easily curable with radiations, and the radiation curing method is extremely favorable to resin compositions comprising it. However, when polypropylenic resin is exposed to radiations, its molecular chains break easily, and the polypropylenic resin cured with radiations is often degraded. Therefore, in general, it is difficult to cure polypropylenic resin with radiations to such a degree that the cured resin is favorably foamed.
Therefore, where polypropylenic resin is formed into foams in a process comprising curing it with radiations, it must be specifically modified so as to be well applicable to the process of curing and foaming it. For example, as in JP-B 46-38716, JP-A 61-69844 and JP-A 5-78514, a polyfunctional monomer compound having a reactive vinyl group, acrylic group or methacrylic group in the molecular structure and serving as a curing agent is added to polypropylenic resin, then the resulting resin composition is exposed to radiations while the radiation energy is well controlled to such a degree that the radiations applied to the resin do not degrade the cured resin, and thereafter the cured resin is foamed under heat into foams.
However, curable and foamable resin compositions that contain the conventional polyfunctional monomer compound serving as a curing agent therein are problematic in that the polyfunctional monomer compound disperses poorly in the compositions and is often localized therein, though depending on the type of the polyfunctional monomer compound used therein and on the constitution of the resin therein. As a result, the resin compositions are cured unevenly. When foamed, the unevenly cured compositions are unstable. In addition, the pores in the resulting resin foams are not uniform in size. The resin foams having such non-uniform pores therein are often defective in their mechanical properties, workability and heat resistance.
On the other hand, when resin sheets are cured with electron beams, the electron beam energy applied thereto shall have a depth-dose-pattern distribution. In that case where the resin sheets contain a conventional polyfunctional monomer compound of which the reactivity is proportional to the radiation energy applied thereto and where they are cured with electron beams, the cured resin sheets shall inevitably have a curing degree distribution in their thickness direction. As a result, the foams from them shall also have a pore size distribution in their thickness direction, correspondingly to the curing degree distribution of the unfoamed sheets. In addition, the other general properties of the foams will also vary in the direction of the thickness of each foam. Owing to these problems, improving the quality of the foams is often difficult.
With the recent development in the techniques of working them, resin foams are much desired to have higher heat resistance and higher pressure resistance. In addition, they are also desired to have high flexibility, though contrary to the heat resistance and pressure resistance. In order to obtain resin foams that satisfy all these requirements, the basic resin compositions for them must be improved. In fact, however, it is difficult to obtain resin foams that satisfy all such requirements for all their applications. In practice, therefore, resin compositions for foams are optimized, depending on the necessary properties of foams as targeted to specific applications. However, the method is disadvantageous to industrial resin foam production, since the number of product types increases and the productivity of such different types of products is low and since the products require much complicated process control and quality control.
On the other hand, aliphatic (meth)acrylate-type polyfunctional monomers may be used as curing agents in producing cured resin foams, but are not so good as having various drawbacks. Concretely, they irritate the skin (PII) and are toxic when taken orally, and therefore they are defective in sanitary safety. In addition, they are hydrolyzable and are therefore poorly waterproof. Owing to such their drawbacks, the monomers require some safety measures when used in industrial resin foam production. Moreover, as their waterproofness is poor, the resin structures as cured with the monomers are often unstable in the subsequent foaming step. For these reasons, the monomers of that type are difficult to use for industrial purposes. In addition, the heat resistance of the resin foams are produced with the monomers is poor.
For polyethylenic resins having a linear structure, ordinary, vinylic functional group-having curing agents such as divinylbenzene are unfavorable, as it is difficult to control the curing degree of the resins with those agents. Concretely, the problems with the curing agents are that the mechanical properties of the resin foams produced with them are often unstable, depending on the type of the functional group in the promoters and on the structure of the promoters themselves, and that the resin foams could not exhibit the designed characteristics. For these reasons, industrial production of foams of such linear polyethylenic resins with the curing agents is not easy.
On the other hand, known is a different method of directly foaming a polyolefinic resin composition that contains an acrylic polyfunctional monomer and a foaming agent, in which the composition is not cured prior to the foaming step. In that method, however, the curing degree of the resin composition is low and practicable resin foams could not be obtained.